August 23, 2016

August's featured paper is titled, "Biosensor reveals multiple sources for mitochondrial NAD(+)," and is published in Science. The paper is published by OHSU authors Xiaolu A. Cambronne, Melissa L. Stewart, DongHo Kim, Amber M. Jones-Brunette, Rory K. Morgan, David L. Farrens, Michael S. Cohen, and Richard H. Goodman.

Nicotinamide adenine dinucleotide (NAD+) is an electron carrier required for producing energy, in the form of ATP, within cells. If mitochondria are considered to be the cell's engine and glucose the fuel, the conversion of NAD+ to NADH is the spark plug, driving the central pathway in glucose metabolism –the Krebs cycle –in a forward direction. Increases in mitochondrial NADH flood the engine and bring the Krebs cycle to a halt, an important control because too much NADH generates reactive oxygen, which is deleterious to the cell. 

Cancer cells generate energy differently, effectively bypassing the mitochondrial contribution, and this metabolic difference between normal cells and cancer cells could provide a target for therapeutic intervention. 

While the metabolic activities of NAD+ have been studied for over a hundred years, its function as a cofactor for enzymes involved in gene expression, genomic stability and neurodegeneration were recognized only recently and have received increasing attention. Indeed, the idea that NAD+ levels decrease with aging, causing a variety of age-related diseases, has generated hundreds of papers and a small industry aimed at increasing NAD+ production.

Testing whether this is a reasonable approach has been hindered by the inability to monitor NAD+ levels within intact cells. Most cellular NAD+ is found within mitochondria, but other pools are located in the cytoplasm and nucleus where they regulate enzymes subserving different functions. Grinding up a cell and measuring NAD+ is inadequate (although frequently done) because of this compartmentalization and also because most of the NAD+ is bound to protein and thereby unavailable for biochemical reactions. Simply put, in the past no one knew how much free NAD+ was available in specific cellular compartments, whether these levels were appropriate to regulate the various NAD+-consumers, or even how the NAD+ got into mitochondria.

A dozen years ago, the lab of Richard Goodman, M.D, Ph.D., senior scientist and principal investigator in the Vollum Institute, began addressing this problem by using fluorescence lifetime measurements, a method that could not determine NAD+ levels directly but nonetheless provided the first glimpses into its concentrations in distinct cellular locations. The team's paper generated some controversy but ultimately was supported by other labs who came to similar conclusions. Still, these corroborating measurements were also indirect and not suitable for answering many of the most important questions.

Michael Cohen, Ph.D., associate professor of physiology and pharmacology in the OHSU School of Medicine, provided the key insight for this month's featured paper by recognizing that a bacterial enzyme, DNA ligase, underwent a significant structural change upon binding to NAD+. 

Lulu Cambronne, Ph.D., research assistant professor in the Vollum Institute, used this information to develop an NAD+ biosensor and, in association with Melissa Stewart, Ph.D., postdoctoral fellow in the Goodman lab, showed that this reagent could report on NAD+ levels within living cells. 

Dr. Cambronne's paper, described by reviewers as "groundbreaking" and "the holy Grail in the field," confirmed the earlier estimates of NAD+ concentrations that were based on fluorescence lifetime measurements. The researchers showed how the different cellular compartments of NAD+ relate to one another and suggested how NAD+ gets into the mitochondria where it can participate in metabolic processes. 

From a practical standpoint, the availability of an NAD+ biosensor may provide insights into mechanisms underlying some models of neurodegeneration, particularly those caused by axon injury. 

Marc Freeman, Ph.D., new Vollum director, discovered that a protein called Sarm1 is activated in this setting and others have shown that NAD+ precursors protect against the ensuing degeneration. 

The labs of Drs. Goodman, Cohen and Cambronne have already begun collaborations with Dr. Freeman to sort out exactly how the NAD+ decrease relates to neurodegeneration in fly and mouse models in the hope that a better understanding of the metabolic derangements in injured axons will lead to novel therapeutic approaches. 

Dr. Cambronne's studies may also shed light on the re-wiring of metabolic pathways in cancer cells, known as the Warburg effect, and provide insights into how to interfere with these pathways without harming normal cells. 

Of final interest, Dr. Cambronne and Warburg have a "Kevin Bacon number" (degree of separation) of five. Otto Warburg discovered NAD+, and his most famous student, Hans Krebs, identified the Krebs cycle. Krebs trained Hans Kornberg, the former biochemistry chair at Cambridge University, who introduced Dr. Goodman to biochemical research. Thus, Dr. Cambronne's current paper is the latest addition to a lineage that dates back to the 1930s.

Resources

• Read the paper
  
• Read the OHSU Research News article
  
• Read about the Goodman lab
  
• Read about the Cohen lab
  

Citation

Biosensor reveals multiple sources for mitochondrial NAD⁺.Science 2016 Jun;352(6292):1474-7Xiaolu A Cambronne, Melissa L Stewart, DongHo Kim, Amber M Jones-Brunette, Rory K Morgan, David L Farrens, Michael S Cohen, Richard H Goodman

More Published Papers 

• Previous School of Medicine Papers of the Month
• Recent OHSU published papers 

Pictured above, from left to right: Drs. Michael Cohen, Xiaolu Cambronne and Richard Goodman